1. Field of the Invention
This invention relates generally to a hydrogen fuel cell system and, more particularly, to a hydrogen fuel cell system that employs a purge valve for purging a combustible anode exhaust gas from the fuel cell stack, an accumulator for accumulating the purged anode exhaust gas, and a bleed valve for slowly bleeding the accumulated anode exhaust gas from the accumulator in a non-combustible manner.
2. Discussion of the Related Art
Hydrogen is a very attractive source of fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode, typically by a catalyst, to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane (PEM) type fuel cells are a popular fuel cell for vehicles. The PEM fuel cell generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorinated acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The combination of the anode, cathode and membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacturer and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Many fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode charge gas that includes oxygen, typically a flow of forced air from a compressor, and an anode hydrogen gas. An anode exhaust gas and a cathode exhaust gas are discharged from the fuel cell stack. The anode exhaust gas is the hydrogen gas stream applied to the stack minus the hydrogen used by the stack and the cathode exhaust gas is a depleted oxygen stream. The hydrogen in the anode exhaust gas makes it relatively combustible. When the anode exhaust gas is vented to atmosphere, an increased localized hydrogen content develops around the exhaust outlet which could become a combustible mixture, thus causing a possible dangerous situation. Sometimes a pump and return pipe is employed to pump the anode exhaust gas back to the anode input so that the unused hydrogen can be recycled. In other designs, the exhaust gas streams are sent to a tail gas combustor that burns the anode exhaust gas using oxygen from air and/or the cathode exhaust gas. Both the pump design for returning the anode exhaust gas to the input and the tail gas combustor add a level of complexity to the fuel cell system that requires significant system control.
In certain fuel cell designs, lower anode stoichiometric operation is desirable for increased efficiency of the system. In a dead-ended anode design, the anode exhaust is closed off and the amount of hydrogen in the input gas is almost completely consumed by the stack. However, dead-ended anode operation requires a periodic anode purge to minimize nitrogen and water buildup in the stack. If the anode is not periodically purged, then the build up nitrogen and water in the stack would decrease performance of the stack, and eventually cause it shut down. The purged anode exhaust gas includes hydrogen that is usually vented to the atmosphere or sent to the tail gas combustor.